1. Field of the Invention
This invention relates generally to optical alignment, and more particularly to integrated multi-use optical alignment for interferometric and reflectometer based endpoint and optical metrology systems.
2. Description of the Related Art
In the manufacture of semiconductor devices, such as integrated circuits or flat panel displays, the substrate (e.g., the wafer or the glass panel) may be processed in a plasma processing chamber. Processing may include the deposition of layers of materials on the substrate and the selective etching of the deposited layer(s). To prepare a layer for etching, the substrate surface typically is masked with an appropriate photoresist or hard mask. A plasma is formed from an appropriate etchant source gas during etching to etch through regions unprotected by the mask. The etching is terminated once it is determined that the target layer is etched through or has reached a predetermined thickness. This termination of the etch is typically referred to as the etch “endpoint.”
Many techniques have been employed in the art to determine when to terminate an etch. For example, in interferometry, a light beam reflected off the substrate is monitored and an etching depth is determined by counting maxima and minima in the amplitude of the reflected beam or from cessation of the signal. The constructive and destructive interference occurs because the light beam is partially reflected off the substrate surface and partially reflected off underlying interfaces. If the original thickness of the layer is known, a remaining thickness may be estimated by counting the maxima/minima peaks during etching.
Interferometric and reflectometer based endpoint and optical metrology systems on semiconductor processing tools typically rely on the accurate alignment of a beam of optical radiation with the normal to the surface of a wafer under test. When optimal alignment is achieved, the incident bean is reflected back into the transmission/collection optics of the optical system and the maximum possible signal strength is achieved.
In order to achieve this alignment, some form of alignment procedure is performed that involves placing a reference wafer on the electrode of the processing system and adjusting the alignment of the optics until the back-reflected signal is at a maximum. For example, FIG. 1 is a diagram illustrating a prior art etch system 100 utilizing an optical endpoint detection system. The etch system 100 includes a process chamber 102 having a wafer chuck 104 and a view port 106. The view port 106 allows an optical beam 110 to be transmitted onto the surface of a reference wafer 112 using a beam-forming system 108.
To properly align the optical beam 110 emitted from the beam-forming system 108, the reference wafer 112 is placed onto the wafer chuck 104. The optical beam 110 then is emitted onto the surface of the reference wafer 112 and the signal reflected is back to the beam-forming system is examined. Broadly speaking, the beam-forming system 108 is adjusted until the reflected is maximized at all wavelengths being utilized. Unfortunately, this procedure interrupts normal process flow because the reference wafer 112 must be loaded into the processing chamber.
Another type of alignment step also is required while a tool is being brought into production or serviced. The robot arm that transfers wafers into the processing chamber should be aligned accurately relative to the position of the electrode within the chamber. This alignment required complex test fixtures and careful measurement. For example, in one process, a reference plate is placed on the wafer chuck 104 and corresponding reference plate is positioned on the robot arm. The robot arm then is manually adjusted until the robot arm is aligned with the reference plate. The final coordinates thereafter are programmed into the robot arm control system such that the robot knows the location of the center of the wafer chuck 104. To perform this operation, both the transport module where the robot operates and the process chamber have to be vented in order to properly position the reference plates. Because it is a time consuming process, re-alignment of the robot arm generally is only repeated if wafers are broken or otherwise obviously misplaced on the electrode.
In view of the foregoing, there is a need for an optical alignment system that allows optical alignment without requiring the normal process flow to be interrupted. In addition, the system should allow proper robot alignment without requiring venting of the process chamber and transport module.